Secured debt is backed up by an asset. The simplest example is a mortgage; if you cannot meet your mortgage payments, the lender will take the house and sell it to recover the money.
Unsecured debt is not backed by any asset. These can include credit cards, council tax or overdrafts. There is no collateral, such as a house, if the repayments are not met. This means creditors usually have to take legal action to regain any money they are owed.